1. Field of the Invention
The present invention relates to a recording material which includes a substrate, a radiation-sensitive layer which contains a 1,2-naphthoquinone-2-diazide and an organic, polymeric binder which is insoluble in water but soluble in aqueous alkaline solutions, and a non-continuous matte layer present thereon.
2. Description of Related Art
The first step in the production of lithographic printing plates is usually the imagewise exposure of the radiation-sensitive recording materials containing quinonediazide in a vacuum contact printing frame. The original is to be brought into close contact with the recording material as rapidly as possible. After evacuation, no air should remain between the original and the recording material. Similarly, the nitrogen liberated from the quinonediazide during exposure should be removed as rapidly as possible so that no nitrogen pockets form. Both requirements can be met with a recording material whose radiation-sensitive layer is roughened on the surface. The rough surface can be achieved, for example, by dusting prior to imagewise exposure. However, such a process generally produces irregularly roughened surfaces, so that irregular copies are also obtained.
The manufacturers of radiation-sensitive recording materials have therefore made efforts to provide their products with a surface having a defined roughness from the start. Various processes were used. One example is that the substrate can be coated with a radiation-sensitive solution in which water-soluble particles are suspended, which are washed out with water after drying of the layer. The surface of the radiation-sensitive layer then has a large number of indentations.
It has also been proposed to add to the coating solution a compound which eliminates gaseous nitrogen on heating to 80.degree.-300.degree. C. Nitroso, sulfonylhydrazine, azo or hydrazo compounds are particularly suitable. The elimination process starts during drying in the above-mentioned temperature range. The radiation-sensitive layer is provided with a rough surface in this manner.
Finally, it has also been proposed to use a radiation-sensitive coating solution having a resin dispersed therein, which flocculates during drying and is precipitated and thus improves the surface structure. However, none of these processes has become established to date.
A rough surface can also be achieved by applying continuous or discontinuous matte layers. Thus, DE-A 30 09 928 (equivalent to GB-A 2 046 461) describes a process in which a photosensitive layer is coated with a non-photosensitive coating liquid which contains, in dispersed form, particles of polyethylene, polypropylene, ethylene/propylene copolymer, ethylene/vinyl acetate copolymer, polyethylene terephthalate or a crosslinked vinyl polymer. After drying, individual particles remain as very small projections in the continuous top layer.
Continuous or discontinuous top layers having a defined surface roughness can be applied with the aid of an embossing roll. The coating solutions used for this purpose may contain conventional matting agents, such as silica, zinc oxide, titanium dioxide, glass beads, alumina, starch, poly(methyl methacrylate), polystyrene and phenol resins (DE-A 26 06 793 which is equivalent to GB-A 1 542 131).
Finally, a matte surface can also be produced by applying a fine powder which is fused with the surface of a photosensitive layer. For this purpose, the powder must have a softening point which is lower than that of the photosensitive layer. The matting particles generally comprise polyvinyl acetate, polyvinylidene chloride, polyethylene oxide, polyethylene glycol, polyacrylic acid, polymethacrylic acid, polyacrylamide, polymethacrylamide, polybutyl acrylate, polystyrene, polyvinyl methyl ether, epoxy resins, phenol resins, polyamides or polyvinylbutyral (DE-A 30 03 131 equivalent to U.S. Pat. No. 4,288,521).
DE-A 31 31 570 (equivalent to GB-A 2 081 919) discloses a recording material having a photosensitive layer and a matte layer present thereon. The matte layer is applied by spraying on and drying an aqueous solution or dispersion which contains an organic copolymer. Spraying can also be effected electrostatically. The radiation-sensitive layer may contain a 1,2-naphthoquinone-2-diazide and a polymeric binder. As shown in Example 1 of the publication, the matte layers must be capable of being removed during development. A matte layer having copolymers which contain 12% by weight or more of polymer units having salt groups meets this condition. If, however, a copolymer having only 4% by weight of sodium acrylate units is used, the matte material is no longer completely removed, and discoloration (spot formation) therefore occurs during printing.
According to EP-A 0 344 985, the material which gives the discontinuous matted layer is dissolved or dispersed in a hydrocarbon having a certain low conductivity and is sprayed electrostatically on the radiation-sensitive layer. The material used for producing the matted surface may also be photosensitive. The advantage of the process is that the droplets striking the layer have a substantially uniform size.
DE-A 34 33 247 (equivalent to U.S. Pat. No. 4,842,982) describes a radiation-sensitive recording material in which a rough top layer is sprayed onto the radiation-sensitive layer and dried. Both layers have essentially the same composition. If the recording material is exposed to light briefly, as may be necessary for fine screen work, residues of the layer frequently remain in the nonimage parts after development. The residues must then be removed in an additional correction step.
A further possibility for producing a matte photosensitive recording material is to introduce into the photosensitive layer finely divided particles whose smallest dimension is at least as great as the thickness of the layer. The particles may be inorganic or organic (DE-A 29 26 236, which is equivalent to AU-B 59 417/80). Thermally cured phenol resins are described as being particularly suitable (DE-A 31 17 702, which is equivalent to GB-A 2 075 702).
JP-A 57-115 548 discloses a recording material whose radiation-sensitive layer contains hydrophobic silica particles having a mean diameter of less than 500 .mu.m and matting particles comprising polyethylene, polypropylene, ethylene/propylene copolymers or crosslinked vinyl polymers having a mean diameter of from 3 to 40 .mu.m. The SiO.sub.2 particles serve for dispersing the matte producing particles.
A recording material in which the radiation-sensitive layer itself is matte has the advantage that, after imagewise exposure and subsequent development, the matte material is retained in the image parts. The separate matte layers described are soluble or dispersible in the developer and are virtually completely removed. However, haloing may occur around the matte producing particles in the radiation-sensitive layer. Furthermore, the matte producing particles may form agglomerates.